This invention relates to an infrared thermometer with a heatable probe head, in particular a clinical thermometer for taking a patient""s temperature in the ear, as well as to a heatable protective cover.
From U.S. Pat. No. 3,491,596 an ear thermometer having a heatable probe head is known. The probe head has at its forward end a cavity receiving a heater element and a field effect transistor serving as heat sensing device. The walls of the cavity are made of a material which is a good heat conductor as, for example, copper. The field effect transistor is thermally coupled to the cavity walls. Radiant heat energy impinging upon the probe head from outside is transferred to the field effect transistor which responds by issuing a corresponding temperature measurement signal. The field effect transistor is preheated by the heater element to a temperature corresponding to the approximate body temperature. This is intended to reduce the response time by comparison with an unheated heat sensing device.
From U.S. Pat. No. 4,602,642 an ear thermometer is known whose probe head includes a waveguide which extends from a forward end of the probe head into the interior of the probe head. At its rear end the waveguide is secured to a metal housing in which a thermopile infrared detector is arranged. The metal housing and the infrared detector connected therewith in a good heat conducting relationship may be heated by heating resistors and a control arrangement to a temperature corresponding to the approximate body temperature. However, for heating such a large part of the thermometer, relatively long heating up periods and correspondingly high amounts of energy are necessary.
By heating the probe head insertable into an auditory canal, a heat flow between the probe head and the auditory canal is largely avoided, that is, the probe head and the auditory canal are in thermal equilibrium during temperature measurement. In this manner, erroneous readings can be avoided which are otherwise introduced by the ear canal""s cooling because the probe head introduced into the ear canal has a lower temperature than the ear canal, resulting in a temperature reading which is too low. Such measurement errors depend not only on the initial temperature difference between the probe head and the ear canal and the cooling period or the duration of the measurement process, but are also influenced by the respective positioning or alignment of the probe head inside the ear canal.
When the probe head is pointed directly at the wall of the auditory canal, the infrared radiation emitted by the cold probe head is partially reflected by the wall (reflection factor 3 to 5%, approximately) and detected by the probe head, as a result of which not only the infrared radiation emitted by the cooled off wall of the auditory canal is measured but also the reflected infrared radiation emanating from the probe head itself, resulting in a temperature reading which is correspondingly too low. By contrast, however, when the probe head is properly pointed at the tympanic membrane, the radiation emitted by the cold probe head is first multiply reflected in the auditory canal with corresponding losses before it is again coupled into the probe head, resulting in a correspondingly reduced corruption of the measurement results.
A further disadvantage of an infrared thermometer with an unheated probe head resides in that the temperature of a protective cover mounted over the probe head is subject to relatively severe variations depending on the difference between the ambient temperature and the ear canal temperature as the probe head is introduced into the ear canal. This causes also a variation of the radiant energy emitted by the protective cover, introducing a corresponding measurement error. If, however, the protective cover is heated, the temperature of the protective cover varies hardly at all during a measurement operation because it is determined by the temperature of the probe head.
It is an object of the present invention to provide an infrared thermometer having a heatable probe head in which the necessary heating energy is so low that it can be drawn, for example, from a battery present in the thermometer.
According to the present invention, this object is accomplished in that the probe head and/or a protective cover adapted to be fitted over the probe head in a manner known in the art is/are configured in such fashion that only the forward area of the probe head and/or protective cover is heatable to a temperature corresponding approximately to the typical ear canal temperature.
The configuration of an infrared thermometer""s probe head as disclosed in the present invention makes use of the fact that the infrared sensor mounted in the probe head has only a limited field of view. Therefore, it is sufficient for only that part of the probe head to display the same temperature as the ear canal which is in thermal interaction with that part of the ear canal lying in the field of view of the infrared sensor. In a probe head constructed in accordance with the present invention, therefore, practically only the forward end of the probe head, that is, in particular its frontal surface, is designed to be heatable. This relatively small area lends itself to being heated in a highly energy efficient way, and it is preferably thermally insulated from the non heatable part of the probe head.
An infrared thermometer of the present invention has at the forward end of its probe head a radiation inlet zone and a heating element which is preferably electrically heatable. Either the heating element is built into the probe head or a protective cover adapted to fit over the probe head is equipped with a heating element. It is also possible to provide two heating elements one of which is installed in the probe head while the other is connected to a protective cover adapted to fit over the probe head.
The heating element comprises, for example, at least one NTC or PCT resistor or transistor mounted at the forward end of the probe head. However, it is also possible for the heating element to be, for example, a conducting track shaped metallic coating or a coating made of an electrically conductive plastics material which is applied to a radiation inlet window arranged at the forward end of the probe head, the protective cover or the probe head itself and surrounds the radiation inlet zone preferably annularly.
In a preferred embodiment of an infrared thermometer of the present invention, the radiation inlet zone is defined by an infrared transparent window which is heatable by the heating element. This is accomplished particularly simply by a heating wire made of constantan, for example, which is routed around the window and connected thereto in a good heat conducting relationship. The window may be made of a chalcogenide glass, for example, which is transparent to infrared radiation and easily formable. In a particularly sophisticated implementation, the window is made of a semiconductor, in particular silicon, in which an electrically conducting track capable of serving as a resistance heating conductor is formed by doping. The advantage of these embodiments having a heatable window is that the heating element enables the entire forward end of the probe head, that is, the complete frontal surface of the probe head including the radiation inlet zone, to be heated to the desired temperature so that the thermal equilibrium in the ear canal is practically not disturbed by the probe head and erroneous readings attributable thereto are minimized.
An infrared thermometer of the present invention further includes a control device for the heating element which is connected to the heating element and a source of energy, for example, a battery. The control device serves to determine and/or regulate the temperature of the probe head. To this effect, it is connected to a heat flux sensor for detecting the heat flow between the probe head and a user""s ear canal and/or to a temperature sensor arranged preferably at the forward end of the probe head. In a preferred embodiment of an infrared thermometer of the present invention, however, the heating element itself is used as sensor instead of providing additional sensors. The control device then determines from measurable characteristic quantities of the heating element as, for example, the electric resistance, the threshold voltage or the forward voltage, the temperature of the heating element and hence the temperature of the forward end of the probe head.
In an infrared thermometer of the present invention affording particular convenience, the control device controls also the processes of activation and deactivation of the heating element prior to and, respectively, subsequent to performing a temperature measurement. The heating cycle is started automatically no later than when the probe head is introduced into a user""s auditory canal, and it is terminated likewise automatically after the temperature is taken, the actions of insertion and withdrawal being detectable, for example, by a variation of the radiation temperature measured by a radiation temperature sensor connected to the control device, or by a variation of the probe head temperature measured by the temperature sensor, or by a corresponding signal from the heat flux sensor. In this manner error factors are eliminated and an adequately high accuracy of measurement is ensured at all times.
Preferably, to optimize the accuracy of measurement the heat flow between the probe head and a user""s ear canal is determined by means of the heat flux sensor, and the heating output is controlled such as to minimize this heat flow. To maintain the heat flow at a minimum possible level from the outset, the probe head is preferably configured such that thermal capacity and thermal conductivity of the probe head regions contacting the ear canal are as low as possible. Therefore, the surface of these regions is preferably made of plastic.
In a protective cover of the present invention, above all the forward area of the protective cover is heatable by an electric heating element. The protective cover is conically shaped in a manner known in the art, having at its forward end an infrared transmissive radiation inlet zone. The heating element is preferably formed of a conducting track shaped coating of metal or an electrically conductive plastics material surrounding the radiation inlet zone annularly. Energy supply to the heating element is by means of an electrical or electromagnetic connecting device using preferably the battery of the infrared thermometer. This thermometer possesses, for example, suitably arranged contacts making electrical connection with the conducting track shaped coating of the protective cover as it is installed. Alternatively, it is also possible to provide a device for inductive energy transfer to a heating element configured as a short circuit winding.